Many aspects of vision deteriorate as a function of age. However, it is not well understood what factors limit vision in older observers. Previous studies, from our lab and others, have addressed this issue in the context of age-related changes in contrast sensitivity. Those studies demonstrated that decreased sensitivity is due to a reduction in calculation efficiency with age, rather than to changes in equivalent input noise. The present study examined whether similar factors underlie age-related changes in another spatial attribute, namely orientation. Seventeen old and 17 young observers performed a 2-IFC task using 3 cpd Gabor patterns that differed in orientation by 12 degrees. The gratings were embedded in high and low levels of white, Gaussian external noise, and orientation discrimination thresholds were obtained by varying grating contrast. An AVOVA on the log-transformed discrimination thresholds revealed a significant main effect of external noise (thresholds were significantly higher in high external noise) and a Group x Noise interaction. The interaction reflected the fact that discrimination thresholds were significantly higher for old observers than for young observers in the low noise condition, but not in the high noise condition. These results suggest that age-related changes in orientation discrimination are due to age differences in equivalent input noise rather than to differences in calculation efficiency. Thus these results suggest that different mechanisms may underlie the effects of aging on grating detection and orientation discrimination. We currently are conducting experiments to identify the causes of the age-related increase equivalent input noise that affects orientation discrimination.